1. Field of the Invention
The present invention generally relates to an FM (Frequency Modulation) demodulator, and more particularly to an arctangent type FM demodulator using a digital signal processing technique.
2. Description of the Related Art
A digital signal processing technique is applied to a construction of an FM demodulator, so as to realize a miniaturization and a high reliability of the apparatus. As an FM demodulator using such a digital signal processing technique, there is an arctangent type FM demodulator.
In the arctangent type FM demodulator, an AGC (Automatic Gain Control) circuit is equipped at an input stage thereof. An FM signal is inputted to this AGC circuit from a tuner or the like. The AGC circuit performs a variable-controls of the gain to amplify the FM signal in an analog form so that the FM signal has a constant amplitude. Namely, the function of the AGC circuit is to remove the amplitude fluctuation included in the FM signal. Then, the FM signal outputted from the AGC circuit is inputted to an A/D (Analog to Digital) convertor. The A/D convertor samples the FM signal, in which the amplitude level is made constant by the AGC circuit, and converts it into a digital signal. A sampling pulse generator supplies a sampling pulse to the A/D convertor.
When the digital signal corresponding to the FM signal is obtained in this manner, the processes after that is mainly performed by using the digital signal processing technique, so as to generate a digital FM demodulated signal, which corresponds to the FM signal in the analog form, through an arctangent operator and a differentiator for performing a predetermined digital signal process for example.
In this arctangent type FM demodulator, if the AGC circuit is not equipped at the input or prior stage of the A/D convertor, since the amplitude of the FM carrier wave received at a tuner etc., drastically changes depending upon the receiving condition, the value of the digital signal outputted from the A/D convertor drastically changes. Then, an error is generated in the arctangent operator or the differentiator for performing the predetermined digital signal process at the later stage of the A/D convertor. This results in a difficulty in obtaining a digital FM demodulated signal in a good condition.
In this manner, in the arctangent type FM demodulator, it is necessary to equip the AGC circuit at the input or prior stage of the A/D convertor.
However, the AGC circuit has a rather complicated structure and is rather expensive and brings about a complexity and a high cost of the apparatus as a whole, which is a problem.